1. Field of the Invention
The present disclosure relates to a method of manufacturing an aluminum-zinc-based alloy sheet using twin-roll casting and an aluminum-zinc-based alloy sheet manufactured thereby.
2. Description of the Related Art
Recently, in line with the global trend for improving fuel efficiency through weight reductions in transportation equipment parts, the need for lightweight and high strength materials increases. Among these materials, the amount of aluminum (Al) alloys used has rapidly increased due to their excellent castability, processability, mechanical properties, endurance limit, and recyclability.
In particular, among the aluminum alloys, 7000 series aluminum alloys, in which zinc and magnesium are added as main alloying elements, have been mainly used as an aircraft material. However, applications of the 7000 series aluminum alloys as an automotive material and cases for electronic devices have been expanded due to their excellent mechanical properties.
Strength of the 7000 series aluminum alloys increases as the contents of additive elements, such as zinc, are increased. However, since the occurrence of casting defects may be facilitated due to the expansion of a solid-liquid coexistence region and various stages of processing must be undergone due to a decrease in processability, the price of a final material may be high in comparison to a typical steel material.
In order to address the above limitations, various research into decreasing manufacturing costs of the 7000 series aluminum alloys by using a simple manufacturing process has conducted, and one of the research areas is a strip casting method, in which a sheet may be directly produced from a melt. However, the strip casting method developed so far is limited to a technique of manufacturing a low-alloyed aluminum alloy, and a technique of manufacturing a high-alloyed aluminum alloy having excellent strength has not been secured.
An aluminum alloy sheet and a manufacturing method thereof were disclosed in Korean Patent No. 10-0933385 as a prior art related to the manufacturing of an aluminum alloy sheet. Specifically, the method of manufacturing an aluminum alloy sheet was disclosed, in which an aluminum alloy melt is introduced and casted between a pair of rotating twin rolls and thus, an aluminum alloy plate ingot that includes greater than 8 wt % to 14 wt % or less of magnesium (Mg), 1.0 wt % or less of iron (Fe), and 0.5 wt % or less of silicon (Si), and has a thickness ranging from 1 mm to 13 mm is obtained by twin-roll continuous casting. In a method of manufacturing an aluminum alloy thin sheet having a thickness of 0.5 mm to 3 mm by cold rolling of the ingot, casting is performed at an average cooling rate of 50° C./s or more until the center of the plate ingot is solidified after the melt is introduced between the twin rolls. Also, in a subsequent process of heating the plate ingot or the thin sheet to a temperature of 400° C. or more, an average heating rate is 5° C./s or more when a temperature of the center of the plate ingot or the thin sheet is in a range of 200° C. to 400° C., and in cooling the plate ingot or the thin sheet from the temperature greater than 200° C., cooling is performed at an average cooling rate of 5° C./s or more until the temperature reaches 200° C.
However, the aluminum alloy is an Al—Mg-based alloy, i.e., 5000 series aluminum alloys, and description, which is related to the 7000 series alloys having the highest strength among aluminum alloys, has not been disclosed in the prior patent.
Also, an aluminum alloy thick plate and a manufacturing method thereof were disclosed in Korean Patent No. 10-1251235. Specifically, the disclosed method of manufacturing an aluminum thick plate includes performing a melting process, in which an aluminum alloy melt is prepared by melting an aluminum alloy including 3.0 wt % to 9.0 wt % of zinc (Zn), 0.4 wt % to 4.0 wt % of Mg, one or more elements of 0.7 wt % or less of Si, 0.8 wt % or less of Fe, 3.0 wt % or less of copper (Cu), 0.8 wt % or less of manganese (Mn), 0.5 wt % or less of chromium (Cr), 0.1 wt % or less of titanium (Ti), and 0.25 wt % or less of zirconium (Zr), other inevitable impurities and Al as a balance; a dehydrogenation process of removing hydrogen gas from the aluminum alloy melt; a filtration process of removing inclusions from the dehydrogenated aluminum alloy melt; a casting process of manufacturing an ingot by casting the aluminum alloy melt having the inclusions removed therefrom; a hot rolling process of manufacturing a hot-rolled plate by hot rolling the ingot to a predetermined thickness; a slicing process of cutting the hot-rolled plate to a predetermined length in a rolling direction and a width; and a smoothing process of smoothing a surface of the sliced hot-rolled sheet, wherein a thickness of the surface of the hot-rolled plate removed in the smoothing process is in a range of 2 mm to 5 mm for one side.
The aluminum alloy thick plate manufactured according to the prior patent is an aluminum-zinc-based aluminum alloy. However, since the ingot is first manufactured and a process of rolling the ingot is then performed instead of twin-roll strip casting as in the present invention, it may not be economical.
Accordingly, during research into a method of manufacturing an aluminum-zinc-based alloy sheet by twin-roll casting, the present inventors developed a method of manufacturing a defect-free aluminum-zinc-based alloy sheet by using cooling rolls having excellent thermal conductivity and controlling a reduction force by a roll speed, thereby leading to completion of the present invention.